The use of means for varying warp tension for fancy weaving of stylized designs, such as leno patterns, is not new. Among such means are easer-bar, slackener bars, jumper motions, etc. Those means have been operated, however, by negative type dobbies, i.e., they positively pulled the harness frames or heddles upward but the frames were pulled downward by springs. Similarly, the motion of the dobby-controlled easer-bar was negative, i.e., moved positively in one direction by the dobby, but moved in the opposite direction by springs.
Such known means for varying warp tension is satisfactory for old type fly-shuttle looms, but unsatisfactory for rapier type weaving machines which operate much faster. Full speed capabilities of rapier type looms cannot be obtained with negative motion easer-bars, even when such looms are equipped with positive motion dobbies which positively move the harness frames both upward and downward.
Moreover, some known types of easer-bars are in the form of three rollers, two fixed in spaced relation and the third movable between the two fixed rollers. This complicates the construction and renders it more expensive.
Examples of means for controlling warp tensions are disclosed in the following U.S. patents:
Bohan U.S. Pat. No. 2,246,658, June 14, 1941; willis et al. U.S. Pat. No. 2,551,920, May 8, 1951; Kulczycki et al. U.S. Pat. No. 2,589,498, Mar. 18, 1951; Pfarrwaller U.S. Pat. No. 3,125,128, Mar. 17, 1964.
Bohan is exemplary of a whip roll which is automatically raised and lowered to compensate for variations in warp tension, i.e., to maintain constant warp tension in a shedding cycle.
Willis discloses a whip roll which is moved, on movement of the shipper handle of the loom to "off" position, to relax the tension in the warp yarns to prevent "set marks".
Kulczycki et al discloses another type device for performing the same function as the movable whip roll of Bohan, i.e., maintaining constant warp tension during a shedding cycle.
Pfarrwaller discloses still another type of device for performing the same function as the devices disclosed in Willis and Kulczycki.